Near-eye displays (NEDs) include head-mounted displays (HMDs) that may project images directly into a viewer's eyes. Such displays may overcome the limited screen size afforded by other mobile display form factors by synthesizing virtual large-format display surfaces, or may be used for virtual or augmented reality applications.
Near-eye displays can be divided into two broad categories: immersive displays and see-through displays. The former may be employed in virtual reality (VR) environments to completely encompass a user's field of view with synthetically-rendered imagery. The latter may be employed in augmented reality (AR) applications, where text, other synthetic annotations, or images may be overlaid in a user's view of the physical environment. In terms of display technology, AR applications require semi-transparent displays (e.g., achieved by optical or electro-optical approaches), such that the physical world may be viewed simultaneously with the near-eye display.
Near-eye displays have proven difficult to construct due to the fact that the unaided human eye cannot accommodate (focus) on objects placed within close distances, for example, the distance between the lenses of reading glasses to a user's eye when the user is wearing the glasses. As a result, NED systems have conventionally required complex and bulky optical elements to allow the viewer to comfortably accommodate on the near-eye display, which would otherwise be out of focus, and the physical environment.
A conventional solution is to place a beam-splitter (e.g., a partially-silvered mirror) directly in front of the viewer's eye. This allows a direct view of the physical scene, albeit with reduced brightness. In addition, a display (e.g., an LCD panel) is placed on the secondary optical path. Introducing a lens between the beam-splitter and the display has the effect of synthesizing a semi-transparent display located within the physical environment. In practice, multiple optical elements are required to minimize aberrations and achieve a wide field of view for such a solution, leading to bulky and expensive eyewear that has prohibited widespread consumer adoption.
A conventional solution for VR applications is to place a magnifier in front of a microdisplay. For example, a single lens placed over a small LCD panel so that the viewer can both accommodate or focus on the display, despite the close distance, as well as magnify the display, so that it appears to be much larger and at a greater distance.